The present invention relates broadly to a surveying satellite apparatus, and in particular to an apparatus for autonomously updating on-board attitude determination and navigation solutions.
The state of the art of surveying satellites is well represented and alleviated to some degree by the prior art apparatus and approaches which are contained in the following U.S. Patents:
U.S. Pat. No. 3,521,835 issued to Braga-Illa et al on Jul. 28, 1970;
U.S. Pat. No. 3,744,740 issued to Godin et al on Jul. 10, 1973;
U.S. Pat. No. 3,749,914 issued to Terasaki on Jul. 31, 1973; and
U.S. Pat. No. 4,679,753 issued to Landecker on Jul. 14, 1987.
The Braga-Illa et al patent is directed to a control system for synchronous satellites which produces control signals with the coincidence of optical signals from two celestial bodies and an ephemeris corrected time signal corresponding to a segment of an orbit with a programmed on-board impulse rocket response to said signals to maintain orbit synchronism.
The Godin et al patent describes a navigation apparatus for a stellar vehicle on which a pair of stellar detectors are fixedly mounted and their respective optical axes are directed to different stars.
The Terasaki patent discusses an apparatus to control the alignment of the spin axis of a communications satellite with the major axis of an eccentric, near polar orbit. The on-board prediction of the time of apogee passage and autonomous control at apogee for the spin axis alignment with the local vertical is also provided.
The Landecker patent discloses a system for surveying features of a planet in which an earth imaging sensor is alternately directed at the earth and a predetermined star field. The optical detections are converted to electronic signals by a sensor electronics module (332). The outputs of the sensor electronics module corresponding to the times when the sensor is directed toward earth are processed by a signal processor (336), the output of which is a data stream which permits image reconstruction by a ground station computer. The outputs of the sensor electronics module corresponding to the times when the sensor is directed toward the star field are directed through on-board star detection thresholding electronics.
At this time, spacecraft either use ephemeris solutions updated by ground command (e.g., ranging by ground command) or analysis of planet images recorded over time by the sensor and determination of updated navigation predictions using computers on the ground or use external (e.g., Global Positioning System) spacecraft inputs. National Oceanic and Atmospheric Administration (NOAA) currently uses landmarks and has in the past used ranging with its GOES series of weather satellites to predict the location of the spacecraft for about a day in advance. Computers on the ground are used to analyze apparent shifts in the location of the landmarks, determine the spacecraft ephemeris, and geographically grid the sensor data; again, no on-board data processing is done. Similarly, no on-board processing is performed to determine the location of the DMSP weather satellite or geographically locate its sensor pixels. In both of the above cases, it is required that the ground stations remain operational (in conflict with some military system assumptions). Spacecraft location using GPS is possible but requires the survival of a significant part of the GPS constellation as well as the weight, power and cost of adding special GPS receivers to a spacecraft. The current series of DMSP weather satellites has an electro-optical sensor sensitive enough to detect city lights and stars at night, but does not process any of this information on-board the spacecraft in order to automatically determine spacecraft location and attitude.
There is a need to allow a spacecraft to operate autonomously in order to reduce system operations cost as well as to permit military satellites to operate normally without ground control for extended periods of time.
While the above-cited references are instructive, there still remains a need to provide a surveying satellite apparatus which autonomously updates on-board spacecraft attitude and ephemeris solutions. The present invention is intended to satisfy that need.